This document specifies a new opcode in the Secure Shell terminal modes encoding. The new opcode describes the widely used IUTF8 terminal mode bit, which indicates that terminal I/O uses UTF-8 character encoding.

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The Secure Shell (SSH) connection protocol [RFC4254] provides an encoding for terminal modes, used in the "pty-req" channel request type.

A commonly used terminal mode is IUTF8, which indicates that the terminal driver should assume that terminal I/O uses the UTF-8 character encoding [RFC3629]. This is typically used by the kernel's terminal driver on the server to decide how many bytes of input to treat as a single logical character during line editing.

SSH does not currently provide an encoding for IUTF8. This document specifies one.

As specified in Section 8 of [RFC4254], all opcodes in the range 1 to 159 have a single uint32 argument; therefore, the IUTF8 opcode is followed by a single uint32 argument. The value 0 indicates that the IUTF8 mode is disabled, and the value 1 indicates that it is enabled.

As with all other encoded terminal modes, the client SHOULD transmit a value for this mode if it knows about one, and the server MAY ignore it.

The security considerations of [RFC4254] apply. This additional terminal mode encoding is believed to have no security implications differing from the existing set of encoded terminal modes.

Since the IUTF8 terminal mode is intended for use in conjunction with the UTF-8 character encoding, the security considerations of [RFC3629] also apply to systems in which this mode is enabled. In particular, terminal drivers that interpret this bit as enabling UTF-8-aware line-editing behavior should carefully consider how that behavior will treat illegal sequences, overlong encodings, and redundant representations of composed characters (see [UNICODE]).